Morphological aspects of the vasculogenesis and angiogenesis during prenatal edification of the circle of Willis: a review

In the literature, there are many articles reporting anatomical variations of circle of Willis (CoW), defined as those changes that lead to the inability of this anastomotic structure to maintain adequate brain flow. Because there is such a wide variation in the configuration of the CoW, its anatomical variations affect the hemodynamics of blood flow, thus contributing to the development of aneurysms or stroke. As such, we consider that a good knowledge of the embryological development of the constituent arteries of the CoW can shed some light on the causes of the appearance of its anatomical variants. Reviewing literature, we will present the embryological development of the constituting arteries of the CoW and will begin with vasculogenesis and angiogenesis of the vascular system as a whole. Then, we will focus on the embryological development of the internal carotid artery (ICA) and its branches because, starting with the embryological day 24, these arteries are the first vessels that begin to develop to provide the necessary blood for the primitive brain. As the hindbrain increases its volume, a larger amount of nutrients is needed. Because a larger amount of blood is required to be provided by the primitive ICAs, there is a need for arterial capacity development and thus the posterior circulation begin to take shape. At this stage, the posterior circulation consists of a plexiform arterial network that receives blood from the carotid artery through the carotid–vertebrobasilar anastomoses. At the 5–8 mm embryonic stage, these anastomoses begin to regress, and the basilar artery and vertebral arteries become independent of the ICA. We are pointing out on the process of regression of these primitive vessels, emphasizing the fact that their persistence represents the starting point for the appearance of anatomic anomalies of the CoW, which are identified in the adult individuals. In this review, we also present and illustrate some developmental abnormalities of the anterior and posterior parts of the CoW.